LTC3780 datasheet(14/28 Pages) LINER

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LTC3780

3780f

diode of Synchronous Switch B or the Schottky diode,

which is in parallel in with Synchronous Switch B, is used

to discharge inductor current. As a result, some cycles will

be skipped when the output load current drops below 1%

of the maximum designed load in order to maintain the

output voltage.

When the FCB pin voltage is tied to the INTVCC pin, the

controller enters constant frequency Discontinuous Cur-

rent mode (DCM). For Boost operation, Synchronous

Switch D is held off whenever the ITH pin is below a

threshold voltage. In every cycle, Switch C is used to

charge inductor current. After the output voltage is high

enough, the controller will enter continuous current Buck

mode for one cycle to discharge inductor current. In the

following cycle, the controller will resume DCM Boost

operation. For Buck operation, constant frequency Dis-

continuous Current mode sets a minimum negative induc-

tor current level. Synchronous Switch B is turned off

whenever inductor current is lower than this level. At very

light loads, this constant frequency operation is not as

efficient as Burst Mode operation or Skip-Cycle, but does

provide lower noise, constant frequency operation.

FREQUENCY SYNCHRONIZATION AND

FREQUENCY SETUP

The phase-locked loop allows the internal oscillator to be

synchronized to an external source via the PLLIN pin. The

phase detector output at the PLLFLTR pin is also the DC

frequency control input of the oscillator. The frequency

ranges from 200kHz to 400kHz, corresponding to a DC

voltage input from 0V to 2.4V at PLLFLTR. When locked,

the PLL aligns the turn on of the top MOSFET to the rising

edge of the synchronizing signal. When PLLIN is left open,

the PLLFLTR pin goes low, forcing the oscillator to its

minimum frequency.

OPERATIO

INTVCC/EXTVCC POWER

Power for all power MOSFET drivers and most internal

circuitry is derived from the INTVCC pin. When the EXTVCC

pin is left open, an internal 6V low dropout linear regulator

supplies INTVCC power. If EXTVCC is taken above 5.7V, the

6V regulator is turned off and an internal switch is turned

on, connecting EXTVCC to INTVCC. This allows the INTVCC

power to be derived from a high efficiency external source.

POWER GOOD (PGOOD) PIN

The PGOOD pin is connected to an open drain of an internal

MOSFET. The MOSFET turns on and pulls the pin low when

the output is not within ±7.5% of the nominal output level

as determined by the resistive feedback divider. When the

output meets the ±7.5% requirement, the MOSFET is

turned off and the pin is allowed to be pulled up by an

external resistor to a source of up to 7V.

FOLDBACK CURRENT

Foldback current limiting is activated when the output

voltage falls below 70% of its nominal level, reducing power

waste. During start-up, foldback current limiting is disabled.

INPUT UNDERVOLTAGE RESET

The SS capacitor will be reset if the input voltage is allowed

to fall below approximately 4V. The SS capacitor will

attempt to charge through a normal soft-start ramp after

the input voltage rises above 4V.

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